In the production of semiconductor circuit elements, workpieces in the form of semiconductor wafers are generally coated with a photosensitive layer and are exposed by an appropriate optical system effecting image reduction in a pattern determined by a mask so that subsequent development and treatment may establish certain conductive or nonconductive paths on the wafer and/or from circuit elements thereon.
In many cases, circuit elements have already been formed on the wafer before such exposure so that during the exposure a precise positioning of the semiconductor wafer relative to the mask and hence to a reference position in the exposure system is essential for proper correlation of the several circuit arrays.
While the exposure station itself is generally provided with optoelectronic means for effecting the fine adjustment of the orientation of the wafer, e.g. with respect to alignment marks provided on the latter (see my copending application Ser. No. 197,991, filed Oct. 10, 1980, and Ser. No. 265,549, filed May 20, 1981), the field of such optoelectronic means is generally extremely limited so that the wafer must be fed to the exposure device with a predetermined orientation designed to ensure that the reference mark or indexing mark will lie within the window of operability of the adjustment means in the photoprinting stage.
This can be achieved, as pointed out in the aforementioned copending application, by providing a positioning stage ahead of the photoprinting stage at which a succession of wafers individually are prepositioned with respect to a reference position, e.g. a grid, and from which they are then transferred to the photoprinting stage.
Positioning stations of the type described, with which the present invention is concerned, are also important for other purposes although their primary utility is in imparting to a semiconductive wafer a predetermined orientation.
The exposure stage, at which the light for exposing the photosensitive layer is projected onto a photosensitive layer of the wafer, is generally provided for X-Y adjustability, i.e. is a table movable in two mutually orthogonal horizontal directions, one of which is referred to as the X-direction while the direction perpendicular thereto is referred to as the Y-direction.
In an earlier positioning device for such wafers, the preadjustment is effected exclusively by acting upon, or in response to, the position of the outer contour of the wafer. In other words, the outer contour of the wafer is positioned with respect to a fixed location on the device.
While this may not be a problem when the exposure system is designed to provide a pattern on the wafer which is subsequently the basis for further manufacturing processes, a problem is encountered if the wafer has previously been provided with circuit elements, for example, since the positions and orientations of these elements may not be precise relative to the outer contour. As a result, in spite of every effort to accurately position the wafer by use of its outer contour, the positions of elements on the wafer may be inexact and, indeed, sufficiently imprecise as to preclude using the exposure device subsequently.